Color conversion is conducted for converting color data of input images when images are displayed by means of three primary colors of red, green and blue because color reproducibility of images differ depending on the characteristics of the display device. The color conversion apparatus described in Japanese Patent Kokai Publication No. 2000-287074 generates calculation terms which are effective for a specific hue or inter-hue region from color data of red, green and blue, and generates converted color data by means of matrix calculation multiplying the calculation terms by predetermined matrix coefficients. The matrix calculation for the color conversion is represented by the following formula (1):
                              [                                                    Ro                                                                    Go                                                                    Bo                                              ]                =                                            (              Eij              )                        ⁡                          [                                                                    r                                                                                        g                                                                                        b                                                              ]                                +                                    (              Fij              )                        ⁡                          [                                                                                          c                      ×                      m                                                                                                                                  m                      ×                      y                                                                                                                                  y                      ×                      c                                                                                                                                  r                      ×                      g                                                                                                                                  g                      ×                      b                                                                                                                                  b                      ×                      r                                                                                                            h1r                                                                                        h1g                                                                                        h1b                                                                                        h1m                                                                                        h1y                                                                                        h1c                                                                                        h2ry                                                                                        h2rm                                                                                        h2gy                                                                                        h2gc                                                                                        h2bm                                                                                        h2bc                                                              ]                                                          (        1        )            
In the formula (1), r, g and b denote calculation terms corresponding to color components of red, green and blue. m×y and h1r; y×c and h1g; c×m and h1b; b×r and h1m; r×g and h1y; and g×b and h1c are calculation terms which are respectively effective for hues of red, green, blue, magenta, yellow and cyan. h2ry, h2yg, h2gc, h2cb, h2bm and h2mr are calculation terms which are respectively effective for inter-hue regions of red-yellow, yellow-green, green-cyan, cyan-blue, blue-magenta and magenta-red. Inter-hue region “red-yellow” for example means the region which exists in an inter-hue zone from red to yellow.
For instance, if the hue of yellow of the input image is to be converted to “reddish yellow,” a calculation term effective for the hue yellow is subtracted from the calculation term g corresponding to the green component. If the hue of green is to be converted to “bluish green,” a calculation term effective for the hue green is added to the calculation term b corresponding to the blue component. Moreover, by adding or subtracting calculation terms effective for a certain inter-hue region in an inter-hue zone yellow-green, to or from calculation terms r, g and b corresponding to red, blue, and green components, desired color can be displayed.
FIG. 1 shows the functions of each calculation term in the matrix calculation represented by the formula (1). FIG. 1 schematically illustrates the calculation terms r, g and b on the assumption that the matrix coefficients Eij in the formula (1) constitute a unit matrix. “Δh1y” schematically represents a calculation term effective for hue yellow, and “Δh1g” schematically represents a calculation term effective for hue green, “Δh2yg1,” and “Δh2yg2” schematically represent calculation terms effective for inter-hue regions in the inter-hue zone yellow-green. Δh2yg1 is subtracted from the calculation term r, while Δh1y is subtracted from the calculation term g, and Δh1g and Δh2yg2 are respectively added to and subtracted from the calculation term b. By the above-described calculations, the conversion characteristics shown in FIG. 2 is obtained. Ro, Go and Bo in FIG. 2 respectively show conversion characteristics of color data red, green and blue. When the green component is reduced in the hue yellow, “reddish yellow” results as shown in FIG. 2(G). When the blue component is increased in the hue green, “bluish green” results as shown in FIG. 2(B).
The conventional color conversion apparatus has a problem in that the color conversion is associated with reduction in the luminance. That is, as shown in FIG. 2, by reducing the green component in the hue yellow, the luminance is reduced in the inter-hue region of yellow-green. (As shown in FIG. 1, the color component g before the conversion, is maximum from yellow to green.)
Moreover, the conventional color conversion apparatus has a problem in that the color conversion is associated with generation of an achromatic component. FIG. 3 shows another conversion characteristics obtained when the matrix coefficient for each calculation term shown in FIG. 1 is adjusted. According to the conversion characteristics shown in FIG. 3, the luminance is not lowered, but none of R1, G1 and B1 is zero in the inter-hue region yellow-green, so that the achromatic component is generated.
The present invention has been made to solve the problem described above, and its object is to provide a color conversion apparatus and color conversion method for correcting the desired hue, without generating an achromatic component and without lowering the luminance.